The present invention relates to a photoelectric cell, a coating liquid for forming a metal oxide semiconductor film for use in a photoelectric cell and a process for producing a metal oxide semiconductor film for use in a photoelectric cell. More particularly, the present invention is concerned with a photoelectric cell wherein the adsorption and carrying amounts of photosensitizer on a metal oxide semiconductor film are large and the strength of bonding of the photosensitizer to the metal oxide semiconductor film is large with the result that the photoelectric cell exhibits enhanced photoelectric transfer efficiency. Further, the present invention is concerned with a coating liquid for forming a metal oxide semiconductor film for use in such a photoelectric cell and a process for producing a metal oxide semiconductor film for use in such a photoelectric cell.
A photoelectric transfer material is a material from which light energy is continuously taken out as electric energy and a material which converts light energy to electric energy by the utilization of electrochemical reaction between electrodes. When the photoelectric transfer material is irradiated with light, electrons are generated from one electrode. The electrons move to a counter electrode, and the electrons having reached the counter electrode return by moving as ions through an electrolyte to the one electrode. This energy conversion is continuously carried out, so that it is utilized in, for example, a solar cell.
The common solar cell consists of an electrode formed by coating the surface of a support such as glass plate with transfer conductive film, wherein semiconductor for photoelectric transfer material is formed on the transfer conductive film, a counter electrode formed by coating the surface of a support such as a glass plate with another transparent conductive film, and an electrolyte sealed between these electrodes.
When this solar cell is irradiated with sunlight, the photosensitizer incorporated therein absorbs visible radiation region to thereby excite the electrons of photosensitizer dye. The excited electrons move to the semiconductor for photoelectric transfer material, and then the transparent conductive glass electrode, further move to the counter electrode. The electrons having reached the counter electrode reduce the oxidation-reduction system present in the electrolyte. On the other hand, the photosensitizer having caused electrons to move to the semiconductor becomes oxidized form. This oxidized form is reduced by the oxidation-reduction system (which becomes reduced form) of the electrolyte to thereby return to the original form. In this manner, electrons continuously flow. Therefore, functioning as the solar cell comprising the semiconductor for photoelectric transfer material can be realized by virtue of the continuous flow of electrons.
A semiconductor having a surface on which a photosensitizer exhibiting absorption in visible radiation region is adsorbed is used as such a photoelectric transfer material. For example, Japanese Patent Laid-open Publication No. 1(1989)-220380 describes a solar cell comprising a metal oxide semiconductor and, superimposed on a surface thereof, a layer of a photosensitizer such as a transition metal complex. Further, Published Japanese Translation of PCT Patent Applications from Other States, No. 5(1993)-504023 describes a solar cell comprising a titanium oxide semiconductor layer doped with metal ions and, superimposed on a surface thereof, a layer of a photosensitizer such as a transition metal complex.
In these solar cells, for increasing the photoelectric transfer efficiency, it is important that the move of electrons from the photosensitizer layer having absorbed light and having been excited to a titania film be performed rapidly. When the move of electrons is not performed rapidly, the electrons recombine with the transition metal complex such as a ruthenium complex to thereby invite the problem of causing a drop of photoelectric transfer efficiency. Therefore, studies, such as on the improvement of the condition of bonding of the photosensitizer to the titania film surface and the improvement of electron mobility within the titania film, are performed.
For improving the condition of bonding of the photosensitizer to the titania film surface, for example, it has been proposed to, at the time of forming a metal oxide semiconductor film, repeat the operations of application of titania sol to a base, drying and annealing to thereby form a porous thick film, thus rendering the semiconductor film porous, with the result that the amount of Ru complex carried on the surface is increased. Further, it has been proposed to carry out an annealing at 400xc2x0 C. or higher to thereby sinter fine particles of titania with the result that a conductivity enhancement is realized. Still further, in Published Japanese Translation of PCT Patent Applications from Other States, No. 6(1994)-511113, it is carried out in order to increase the effective surface of the semiconductor to, after the formation of a semiconductor layer composed of a porous titanium oxide, immerse the semiconductor layer in an aqueous solution of titanium chloride, or to electrochemically deposit titanium oxide on the porous titanium oxide semiconductor layer with the use of a solution of titanium chloride hydrolyzate.
However, in the current state of the art, there are problems, for example, when annealing is effected for increasing the electron mobility, sintering occurs to thereby decrease the porosity (effective surface) with the result that the adsorption amount of photosensitizer is lowered. Moreover, because the photoelectric transfer efficiency is not satisfactory, the use thereof is limited. Therefore, there is a demand for further improvement of the solar cells.
An object of the present invention is to provide a photoelectric cell wherein the adsorption proportion of photosensitizer is high, the reactivity of photosensitizer is high, electron, movement is smooth within the semiconductor and, hence, the photoelectric transfer efficiency is enhanced. Another object of the present invention is to provide a coating liquid for forming a metal oxide semiconductor film for use in such a photoelectric cell. A further object of the present invention is to provide a process for producing a metal oxide semiconductor film for use in such a photoelectric cell.
A photoelectric cell of the present invention (hereinafter referred to as xe2x80x9cthe first photoelectric cellxe2x80x9d) comprises:
an insulating base having on its surface an electrode layer (1), the electrode layer (1) having on its surface a metal oxide semiconductor film (2) on which a photosensitizer is adsorbed;
an insulating base having on its surface an electrode layer (3), the electrode layer (1) and the electrode layer (3) arranged opposite to each other; and
an electrolyte sealed between the metal oxide semiconductor film (2) and the electrode layer (3),
wherein:
at least one of the electrode-having insulating bases is transparent; and
the metal oxide semiconductor film (2) comprises anatase titanium oxide particles.
It is preferred that the anatase titanium oxide particles have a crystallite diameter ranging from 5 to 50 nm. It is also preferred that the anatase titanium oxide particles be colloid particles having an average particle diameter ranging from 5 to 600 nm. These anatase titanium oxide particles are preferably those obtained by subjecting peroxotitanic acid to heating and aging.
Another form of photoelectric cell of the present invention (hereinafter referred to as xe2x80x9cthe second photoelectric cellxe2x80x9d) comprises:
an insulating base having on its surface an electrode layer (1), the electrode layer (1) having on its surface a metal oxide semiconductor layer (2) on which a photosensitizer is adsorbed;
an insulating base having on its surface an electrode layer (3), the electrode layer (1) and the electrode layer (3) arranged opposite to each other; and
an electrolyte sealed between the metal oxide semiconductor layer (2) and the electrode layer (3),
wherein:
conductive protrusions (4) jutting from the surface of the electrode layer (1) exist, the metal oxide semiconductor layer (2) formed so as to cover the conductive protrusions (4) and the electrode layer (1), and
at least one of the electrode-layer-having insulating bases is transparent.
It is preferred that the metal oxide semiconductor layer be formed along a contour of the conductive protrusions (4). It is also preferred that the metal oxide semiconductor layer comprise spherical particles of at least one of metal oxides selected from the group consisting of titanium oxide, lanthanum oxide, zirconium oxide, niobium oxide, tungsten oxide, strontium oxide, zinc oxide, tin oxide and indium oxide. These spherical particles preferably have an average particle diameter ranging from 5 to 600 nm.
In particular, the spherical particles are preferably those composed of anatase titanium oxide or those having a core-shell structure comprising a core particle of 0.1 to 500 nm in average particle diameter having its surface covered with a shell. It is preferred that the shell of the spherical particles of the core-shell structure be composed of anatase titanium oxide. This anatase titanium oxide is preferably one obtained by subjecting peroxotitanic acid to heating and aging as mentioned above.
The metal oxide semiconductor layer of each of the first photoelectric cell and second photoelectric cell according to the present invention preferably contains a titanium oxide binder. This metal oxide semiconductor layer is preferably one obtained by implanting ions of at least one gas selected from the group consisting of O2, N2, H2 and inert gases of Group 0 of the periodic table and thereafter annealing. Further, it is preferred that the metal oxide semiconductor layer have a pore volume of 0.05 to 0.8 ml/g and an average pore diameter of 2 to 250 nm.
The coating liquid for forming a metal oxide semiconductor film for use in a photoelectric cell according to the present invention comprises peroxotitanic acid, anatase titanium oxide particles and a dispersion medium. The anatase titanium oxide particles are preferably those obtained by subjecting peroxotitanic acid to heating and aging.
The process for producing a metal oxide semiconductor film for use in a photoelectric cell according to the present invention comprises applying the above coating liquid for forming a metal oxide semiconductor film for use in a photoelectric cell to thereby obtain a coating film and hardening the coating film. In this process for producing a metal oxide semiconductor film for use in a photoelectric cell, the hardening of the coating film is preferably performed by irradiating the coating film with ultraviolet light. The coating film, after the hardening by ultraviolet light irradiation, may be subjected to implantation of ions of at least one gas selected from the group consisting of O2, N2, H2 and inert gases of Group 0 of the periodic table and thereafter annealing.